In the semiconductor industry and other industries, there is need for precise flow control or reactants and other chemicals. For example, in chemical vapor deposition (CVD) processes, there are demanding requirements for process control, particle generation control, reliability improvements and cost control (See, e.g., "Vapor delivery methods for CVD: An equipment selection guide," Solid State Technology, May 1996, p. 91). Some of the compounds used as precursors are corrosive to many materials used in the delivery system. Most of controlled delivery of gas-phase materials is accomplished using thermal mass flow controllers (MFCs) and control valves which are electromagnetic ("thermal needle" or solenoid), piezoelectric or lever design. An example of a lever or pivot arm valve which can be used with corrosive materials is disclosed in U.S. Pat. No. 5,314,164. In the past 10 years, solenoid and piezoelectric type valves have been the design of choice as the need for more and more precise control of gas flow in industry has become the rule.
In the past two years, niche industries have been progressing towards even more accurate gas delivery via the mass flow controller. Flow control command speed, accuracy, linearity across a broad range of flow, and repeatability are all prerequisites to more accuracy. In order to achieve these more precise delivery requirements, valve response speed and flow range resolution have to be maximized and oscillation settling time must be minimized. This movement has dictated that such MFCs now operate under digital resolution electronics instead of their classical analog versions. With this new movement to digitally controlled MFCs, to date only the piezoelectric proportionally controlled valve has shown any promise of providing the control requirements needed.
The current "state of the art" piezoelectric control valve for digital mass flow controllers, usually with a combination (stack) of piezo elements for motion range, can control flow over a range of 0-30 slm of nitrogen gas flow. The number of crystalline piezo elements employed determines the full range of compliance, stiffness, motion, capacitance, and nonlinearity.
Piezoelectric elements are very good capacitors, as opposed to thermal or solenoid valves which are not very efficient energy devices, and which lose energy through heat dissipation. In fact, piezoelectric elements retain an internal resistance on the order of 1.times.10.sup.11 ohms. Thus, under static operation of a piezo element, virtually no current is drawn, nor power consumed, to maintain an activation state and/or position.
A term known as "creep" is an inherent part of piezoelectric actuated valves. Creep is referred to as the physical attribute of the piezo material whereby an initial step change in voltage will produce an initial response (motion) in a fraction of a millisecond, followed by a smaller change on a longer time scale. The longer term change is always in the same direction as the initial dimensional change. Creep is an inherent function of piezo elements which causes overshoot and undershoot of a commanded flow setting via the mass flow controller. If the creep is known and reproducible, it can be corrected for through a well-known table-look-up procedure. The long-term movement usually ends within about 0.3 seconds.
With piezo element valves, power dissipation is minimized but some error from hysteresis is never eliminated. Power dissipation is referenced to the tangent of the loss angle for the piezo material. Power dissipation factor is actually the measure of the "breadth" of the hysteresis loop. Hysteresis is the difference in the strain that occurs when a particular voltage is approached whether from a high or low state. A hysteresis curve is generated by plotting power dissipation extension as the applied voltage is increased from zero to maximum voltage, and back from maximum to zero voltage. Although hysteresis can be compensated for in proper digital algorithm control software (usually referenced as PID), some error is never recovered.
In addition to hysteresis errors, non-linearity of piezo elements also contributes to flow errors. Linearity is usually specified for the lower (increasing voltage) part of the curve. Linearity is defined as the maximum percent deviation of any point on the curve relative to the best straight least squares fit to the target data/position. This phenomenon occurs only at the lower end of the voltage curve because the higher voltage end of the electric field strength is approaching the limit where no further alignment of the electric dipoles inside the piezo material can occur. Over this lower operating range, it is possible to correct for the quadratic non-linearity by applying a de-rating voltage constant to compensate for some of the material non-linearity. For example, non-linearity is usually less than 7% of full scale flow for a 0-30 slm piezo element control valve.
Additionally, current piezo element control valve technology results in MFC supplier cost higher than that of other valve control technologies because the stringent manufacturing requirements of piezo elements limit the number of suppliers. Also, since the current piezo element is constructed of a piezo crystalline element usually sandwiched between two silver electrodes, operating conditions such as oxidizing, corrosive or high temperatures subject the silver elements to rapid oxidization and catastrophic electrical failure. Therefore, stringent manufacturing procedures must be put into place to assure proper yields and reduce contamination.
Another problem with current piezo technology for digitally addressed mass flow controllers is the ability of the proportional control valve to pinpoint flow rates in a repeatable manner. The goal of future digital mass flow controllers would be to reduce the envelope of flow errors associated with consecutive flow commands at the same rate. Variations which are extremely good for a proportional valve when working in semiconductor processes with geometries of 0.35 micron or more will be less satisfactory for the future processes applied to less than 0.35 micron geometry. More precise flow regimes will be needed.
Valve generated particles within the flow stream of a mass flow controller is a constant concern to the user. Some progress has been made over the past few years to reduce the amount of friction related issues which cause such particles in the gas flow path. The addition of a diaphragm material (usually 316L and/or VIM/VAR) across the seat of the valve orifice isolates most of the moving parts of the piezo element actuator, but not all friction generated particles are eliminated. Another form of particle generation occurs in mass flow controllers when reactive gases are utilized and where a dead volume within the MFC exists.
The use of ferroelectric thin films in microelectromechanical systems is known. The superior piezoelectric properties of ferroelectric ceramics make them ideal for such devices as microactuators and their use in microvalves has also been suggested. ("Ferroelectric thin films in Microelectromechanical Systems applications," MRS Bulletin, July, 1996, p. 59)
What is needed is a flow control valve which can be manufactured at reasonable cost, which can control flow over a wide range of values and do so with highly repeatable results, which can be made for use with a wide range of corrosive gases or liquids and in which generation of particles is minimized.